Back to Basics: Thermowell protection

Thermowells, while protecting temperature sensors from a process fluid, can undergo tremendous stresses. A new standard is designed to lower risk of failures.

Jennifer Wilson

12/27/2011

Thermowells protect temperature sensors from direct contact with a process fluid. But once inserted into the process, the thermowell can obstruct flow around it, leading to a drop in pressure. This phenomenon creates low-pressure vortices downstream of the thermowell (the same principle underlying vortex flowmeters). Vortices can occur at one side of the thermowell and then the other, which is known as alternating vortex shedding. This effect can be seen in the example of a flagpole rippling a flag in the wind.

The result is that thermowells experience a combination of stresses: the flow pushing on the thermowell (drag forces) and the vortex shedding (lift forces). Instrument engineers should evaluate the thermowell to see if it can withstand these stresses as they can cause mechanical failure. The industry standard for this evaluation is ASME PTC 19.3 TW-2010, which, in 2010, superseded ASME PTC 19.3 1974. Motivation for the new standard followed some catastrophic failures of thermowells in nonsteam service. These thermowells had passed the criteria laid out in 1974. The 2010 standard includes significant advances in the knowledge of thermowell behavior, increasing from four pages in 1974 to over 40 pages in 2010. The recent standard evaluates thermowell suitability with new and improved calculations including:

Various thermowell designs including stepped thermowells

Thermowell material properties

Detailed process information

Review of the acceptable limit for frequency ratio

More accurate evaluation of stresses that affect thermowells.

Forces on a thermowell

As mentioned earlier, flow passing the thermowell creates alternating vortices downstream known as shedding vortices. These shedding vortices cause the thermowell to vibrate. If this vortex shedding rate (fs) matches the natural frequency (fnc) of the thermowell, resonance occurs, and dynamic bending stress on the thermowell greatly increases.

Forces created by the fluid in the Y plane (in-line with flow) are called drag, and forces created in the X plane (transverse to flow) are called lift, as shown in Figure 2. The vortex shedding rate for the drag and lift must be calculated. The in-line forces (parallel to flow) are approximately 2x the transverse forces.

Low velocities

If the fluid is flowing at a very low velocity, the forces exerted on the thermowell are small, which greatly reduces the risk of thermowell failure. The new standard states that the natural frequency, frequency limit, steady-state stress, and dynamic stress do not need to be calculated if all the following conditions are met:

The process velocity, V, is less than 0.64 m/s [2.1 ft/s]

Root diameter minus bore diameter (A – d) ≥ 9.5 mm [0.376 in.]

Unsupported length, L ≤ 0.61 m [24 in.]

Root diameter, A ≥ 12.7 mm [0.5 in.]

Tip diameter, B ≥ 12.7 mm [0.5 in.]

Maximum allowable working stress, S ≥ 69 MPa [10 ksi]

Fatigue endurance limit, Sf ≥ 21 MPa [3 ksi]

The thermowell material is not subject to corrosion or embrittlement

Although the risk of thermowell failure is small if these conditions are met, in-line resonance can still be excited at low velocities, which may lead to sensor failure.

Quantitative criteria

There are four evaluations to be carried out on a thermowell at each set of process conditions to determine the suitability.

Frequency ratio—the forces on the thermowell due to the process conditions—shall not allow the thermowell to vibrate at the critical resonance. See the Frequency ratio limit section below for details on this criterion.

Steady stress—combines radial, axial, and tangential stresses due to external pressure with stress caused by drag at the design velocity. This information is then used in the Von Mises criterion, which must be less than 1.5 times the maximum allowable working stress for the thermowell material.

Dynamic stress—the dynamic predicted stress (including drag and lift forces) must not exceed the fatigue stress limit for the thermowell

Pressure stress—the stress put on the thermowell due to the process pressure must not be more than the thermowell is rated to. This evaluation is carried out at the shank and at the tip; both must be rated higher than the process pressure.

Frequency ratio limit

The frequency ratio (fs/fnc) is the ratio between the vortex shedding rate and the installed natural frequency. In the old standard, the frequency ratio limit was set to 0.8. This was to avoid the critical resonance caused by the transverse (lift) forces. Figure 3 shows the transverse resonance band above the 0.8 limit. Following the inclusion of the in-line (drag) forces, a second resonance band (shown in black) also needs to be avoided.

The frequency limit ratio is set at either 0.4 or 0.8. The criteria for which limit to use is defined in ASME PTC 19.3 TW-2010, and the theory is simplified in Figure 4. This is the theory used in the calculation and should not be estimated without carrying out the full evaluation.

Improvements to design

If a thermowell fails the evaluation, the design can be changed in the following ways:

Shorten the thermowell to reduce the unsupported length

Increase the thickness of the thermowell (A and B)

A velocity collar can be added to reduce the unsupported length, although this is not generally recommended. A velocity collar is used to provide a rigid support to the thermowell and will work only if there is an interference fit between the standoff wall and the collar. Care must be taken to ensure the collar meets the standoff wall at installation and is not affected by corrosion. If a velocity collar is the only viable solution, it is the responsibility of the operator to ensure there is an interference fit between the standoff wall and the velocity collar.

- Jennifer Wilson attended the University of Nottingham, achieving a BEng Hons in Chemical Engineering. She has been working for ABB for 5 years following completion of the ABB graduate scheme and has been involved in engineering and design for temperature and differential pressure (DP) products.

Figures 2 and 3 were reprinted from ASME PTC 3 TW-2010, by permission of The American Society of Mechanical Engineers. All rights reserved. No further copies can be made without written permission from ASME.